Silver is known to have the highest reflectance of all of the metals in the wavelength range from about 400 nm through the far infrared. But below about 400 nm the reflectance of silver drops to a minimum at about 320 nm due to optical constants and surface plasmon resonance (see FIG. 3). Because of this low reflectance gap in the 300-400 nm wavelength range, silver metal is typically used only in mirror applications requiring high reflectance down to 400 nm, but not for reflecting in the UV part of the spectrum. Other mirror applications exist, however, which require high reflectance over a wider spectral range than capable by ordinary silver, such as from about 300 nm in the UV to the far infrared regions (˜10000 nm), characterized herein as broadband. Such high reflectance broadband mirrors are important, for example, in astronomical instruments/applications because it maximizes the efficiency over a wide band of wavelengths of interest to astronomical science. For terrestrial mirror instruments and applications in particular, the earth's atmosphere absorbs all UV wavelengths of light below about 280-300 nm, i.e. the cutoff for atmospheric transmission. As such, broadband high reflectance from about 300-10000 nm would enable the reflection of substantially all terrestrially present wavelength ranges.
Furthermore, silver tends to be very soft mechanically and easily abraded, as well as susceptible to tarnishing and corrosion over time from ordinary atmospheric contaminants/conditions, such as by reacting with, for example, oxygen, chlorine, sulfur, and ozone. Because of this silver coatings are often not used in optical mirror systems unless suitably protected from the elements.
U.S. Pat. No. 6,078,425 issued to the Applicant (Wolfe) shows one example of an environmentally durable high reflectance silver mirror for broadband reflection in the range 300-10000 nm. In the '425 patent, aluminum is used for its high reflectance in the UV portion of the spectrum, and a thin layer (350 Å) of silver is placed on top of it. The low reflectance limitation of silver below 400 nm is compensated by the aluminum layer which is highly reflective down to 200 nm in the UV region, while the silver layer compensates for a dip in reflectance of aluminum at 850 nm caused by inter-band transitions. In this manner, aluminum works optically with silver to increase and widen the high reflectance range. Additionally, a stack of durability layers using, for example, metal oxides, operate to protect the silver layer from the elements.
One problem, however, with the arrangement of the '425 patent is that aluminum and silver metals in contact with each other tend to form a galvanic cell, due to the electrolytic effect, and cause deterioration of the coating, especially if pinholes are present in the coating. While placing a NiCrNx layer between the aluminum and silver may help slow down the electrolytic effect and the resulting degradation, long term testing (e.g. about 7 months) conducted by the Applicant in work performed for the Lawrence Livermore National Laboratory, has shown that the degradation of the silver layer continues, especially when pinholes are present.
There is therefore a need for a high reflectance silver mirror over a wide bandwidth, such as from about 300 nm in the UV to about 10000 nm in the far infrared, to maximize optical throughput, and without utilizing aluminum to achieve the increased bandwidth, so as to void forming a galvanic cell. Furthermore, it is also desirable to provide protective barrier layers which help resist corrosion and tarnishing from environmental contaminants or humidity, and are mechanically durable and robust to withstand, for example, the 20 rub eraser test.